Bogen MX-60 Vintage P.A. Amp

[Guitarzan]

Hi guys!
I'm still working on the Maggie, it's in another thread I have going here. I was going to make a big push to get to a point with it today and share that progress with the BB but I ran into an old snag that's been in my way for quite some time. It's taking up some pretty valuable real estate on my bench and needed only (so I thought) a small amount of work but I'm the king of procrastinators so there it's sat for close to 2 years.


The issue with it was that the inputs for  Mic 1 - 4 in the back were of the wrong gender for a modern XLR mic cable to simply plug right into it. I'm curious if anyone can tell me first off why they were made that way back in the day, and second, simply replacing them with the other gender isn't an issue is it? As long as I followed the old scheme of the old connectors. In other words, the pin marked as pin #2 had the red wire on it when it was a male connector so I soldered the red wire to pin #2 on the female connector that I replaced it with. I can't imagine that something so simple could possibly be an issue but I figured better to ask and be corrected than to leave it wired wrong. I didn't have a chance to test it as it would mean moving some pretty heavy stuff around to get it all hooked up, and since we havn't played lately everything's sort of "put away" at the moment.


The other problem that arose at the end of this connector swap-out was that 4 impedance matching transformers that are optional, and seem to be getting hard to find would no longer fit in their sockets because the each respective transformer socket lives right behind the XLR jack inside the top cover and new connectors are just a little longer that the originals so now they're in the way and I can't plug the transformers in. I can shim out the connectors and get them back to where the transformers with fit unhindered right back in their sockets, but while I'm on the subject of those things, do I really need them? I generally use a basic mic. Shure SM-57 or SM-58 depending on if it's for vocals or micing an amp, and occasionally something a little higher end than that, but an SM-58 is usually my basic workhorse mic and I've had a LOT of them over the years. Do I need that impedance matching transformer with an SM-58?


Finally, right next to the impedance matching transformer socket, there a little slide switch that has 2 options, Hi Z and Lo Z. What to these switches do?


Besides shimming those connectors out enough to get the impedance matching transformers back into place, I'm done with that Bogen project and can clear the bench space to devote the entire area for the Weber Maggie project. I've never had to deal with these sorts of things until recently. I was always the guy who just plugged in and played. Never wondered about or concerned myself with the technical aspect of things because the band had a guy who was already a genius at that stuff when we first started back in the late '60s. Now that we're all retired and the band is no more, I have sort of inherited the task of making this stuff go and keeping it going for the occasional jams and I think even those are drying up to the point where I'm just gonna sell off most of this stuff soon. Still, I'm curious and would like to know so any Bogen guys out there, or just old-school tube powered P.A. guys, please fill me in on these questions.


Please and thanks as always and looking forward to seeing the replies.

[jjasilli]

Manuals & schematics for the Shure 57 & 58 can be downloaded from the web.  Memory tells me that you turn a set screw (reverse thread) into the base of the mic body to release a tiny tranny; then switch a jumper to select hi or lo impedance output.  If you don't know the difference, there's lots of info on the web, or re-post.  The output impedance of the mic (or other device) must match the input impedance of the amp.  In summary, a lo impedance mic is "better":  better signal to noise ratio with less feedback.  BUT, hi impedance will plug directly into a tube amp (which has a hi impedance input).  It's also more "traditional", say, for blues harp.


The MX-60 schematic is available on the web.  It has a typical tube amp 1M input impedance, which matches a hi impedance mic.  The impedance SW on the MX-60 either: a) bypasses the input tranny for use with a hi impedance source device; or b) puts incoming signal thru through an input tranny to match a lo impedance source device to the amps' hi impedance input circuit.


If the output impedance of the mic doesn't match the input impedance of the amp, then the signal will be weak, nasally, lacking in frequency response, with lots of early feedback.  It's very obvious to the ear.

[sluckey]

There's always stuff on my bench that's in the way when I start a project. Most times it's not even electronics related. I usually just move it out of the way. Again, and again!   

[jjasilli]

My bench too!

[shooter]

Again, and again

Yep

[Guitarzan]

There's always stuff on my bench that's in the way when I start a project. Most times it's not even electronics related. I usually just move it out of the way. Again, and again!   

Yeah but for TWO YEARS??!? Holy moly man! I though I was bad. I didn't reckon that any damage could be done by not having those transformers plugged in especially considering they're optional to begin with, and that the "Hi Z" "Low Z" swtches wouldn't be much more dangerous. I've just always wondered what they stuff meant/was there for. Many of out mic cables have impedance matching transformers built right into 'em. They have those really long XLR or 1/4" ends on 'em and I was told by our former guru of all things electronic that IMTs were what was in there causing them to be so much bigger than the "normal" ones.


Thanks for filling me in. I really should get a copy of the manual for that amp. One of the guys who has since fallen away like the rest of 'em, had a mixing board for "basement" use that we used with this amp. So I just used one of the RCA inputs (I think) and made an adapter to allow us plug that board into the Bogen. I must say tho, that Bogen is a really great sounding amp for use as a basement P.A. system. Without the board, if and when we set up for a jam next time, We'll have to use the 4 mic inputs which will be plenty at this point but they were the wrong gender which is why we've never used them to this point. I could never figure out why they were set up that way in the first place. Did they used to have mic cable configured the other way 'round back when they build this thing? All 4 of the mic inputs were "Males" and all of my mics are males, as well as the cables that go with them. I mean, they're male on...well you know what I mean.
If they were 1/4" instead of XLR it would be like a 4 phone plugs sticking out of the back of the amp.

[vibrolax]

I agree with jjasili.
1) you need the impedance matching transformers to use low impedance balanced sources like SM57's and SM58's without loss of fidelity and level
2) the Hi-Z switch position bypasses the input TX, the low-Z position goes through the TX

Another minimum effort solution would have been to replace the male XLR end of the mic cables with a female XLR,
or get a few Female-female adapters.

The shim-out solution you chose to use female connectors in the chassis is fine in my opinion.

Jon

[Guitarzan]

I agree with jjasili.
1) you need the impedance matching transformers to use low impedance balanced sources like SM57's and SM58's without loss of fidelity and level
2) the Hi-Z switch position bypasses the input TX, the low-Z position goes through the TX

Another minimum effort solution would have been to replace the male XLR end of the mic cables with a female XLR,
or get a few Female-female adapters.

The shim-out solution you chose to use female connectors in the chassis is fine in my opinion.

Jon

Thanks Jon and to all who helped out. You guys are really the best when it comes to getting questions like these answered in a hurry. I've found some plastic fender washers that I think are for attaching Styrofoam panels to a wall. Anyway, they're just the right O.D. to match up with the jacks which are already in but since the center of the jack comes out, and it's the part that's soldered, this isn't going to be a problem. I can put 2 or 3 at a time in my Greenlee electrician's hole punch for pipe fittings and make the I.D. perfect as well. Then it's just a matter of drilling the 3 little holes and it's finished. I'll have to stack 3 or 4 of them together to get it shimmed out far enough. I'll post some pix when I finish it up. Just cuz you guys were nice enough to reply to this. It's always fun to look at gut shots of peoples equipment. I think it is anyway.

[PRR]

> you need the impedance matching transformers to use low impedance balanced sources like SM57's and SM58's without loss of fidelity and level

"Fidelity" will be fine.

Loss of level is an issue, but depends on the situation.

I used to use low-Z dynamics into guitar amps ALL the time for quickie PA systems.

My rules:

For a soft-spoken talker in a larger room, I would probably need a low/high transformer to get the system to acoustic feedback (all the gain a PA system can use in a room).

For closer and louder sources, musicians in general, things were fine without a transformer.

In extreme cases (mike-eating shouting singers), I could *not* use a transformer, the guitar amp input would overload.

[Guitarzan]

I opened the bottom panel for the first time just to have a peek at what it looks like in there while I'm messing with stuff. I was thinking of putting a 1/4" jack in where the mic 5 input resides right now but it's too tight, clean, and just not as easy change out like I'd hoped so I'm just gonna leave it alone. I have that adapter around here someplace anyway, from the weird round screw-on connector to 1/4" phone jack.


Anyway, I just wanted to comment on how clean and perfect it all looks in there. I'm certain I was the first person ever to back out those screws that hold the bottom panel on since the day it came outta the factory. When I first got it 6 or 7 years ago I didn't have a variac so I used the TV repairman light bulb-in-series device to fire it up and it has never showed any signs of needing anything, ever.


So I kind of hated to "update" it with the new XLR connectors of the right gender, but it needed to be done in order to remain useful, to me anyway. The worst part really is having to shim them out to allow the impedance transformers to fit back in there. The solution I finally came came up with isn't to awful tho. it was a little time consuming to knock out the centers of all of those nylon washers and then have to nibble a notch in them so they fit the "bump out" in the connector body where that release button mechanism goes. They're all done now tho so it's just matter of undoing that set screw to release the center of the connector that's wired to the amp, and pull the bezels out and shim them, then put it all back together again. Shouldn't take more than an hour or so. I'll post a picture of the finished product for your review and invite any critiques you may have to offer. Let me how you'd have done it differently or whatever.

[sluckey]

Why not just leave it original and get some gender changers. They're cheap. Here's a female to female. Males are even cheaper...

http://www.bhphotovideo.com/bnh/controller/home?O=&sku=408143&gclid=CK-Ak6-5ssYCFdgYgQodTL4B9Q&is=REG&m=Y&A=details&Q=

[Guitarzan]

Why not just leave it original and get some gender changers. They're cheap. Here's a female to female. Males are even cheaper...

http://www.bhphotovideo.com/bnh/controller/home?O=&sku=408143&gclid=CK-Ak6-5ssYCFdgYgQodTL4B9Q&is=REG&m=Y&A=details&Q=

Prolly what I shoulda done. I don't remember what I paid for the XLR connectors anymore, but I wouldn't be surprised if it was in the same neighborhood as those adapters. Well, that'll learn me. Next time I'll post what I'm thinking about doing before plowing ahead with it and starting a thread after. There are enough of these things around to where it isn't really a sin to modify one. If it was something really rare and special, I never would have touched it but it is what it is now. I had some odds & ends to take of before heading down to my bench. It's starting to get pretty warm so I guess I managed to get something right today anyway. It's nice and cool down there, and quiet too. Not that it's grand central around here to begin with but the solitude down there is really very nice. So the next post will be with the picture of what I did to the thing.

[jjasilli]

These & similar amps are often modded to modern connections.  Gender changers are an option, but personally I'd opt for your solution.  Personal preference.

[Guitarzan]

These & similar amps are often modded to modern connections.  Gender changers are an option, but personally I'd opt for your solution.  Personal preference.

Yeah, well when I had the idea, originally, I thought it'd be a cleaner fix than gender changers. Then the Impedance trannies became and issue, then I thought I'd simply loosen the set screw and slide the guts out the back of the body and shim the bezel out but it slides out the other way. So...it means de-soldering all of the connections, putting my shims in and the re-soldering everything. Murphy's hard at work on this one. So I got 2 of 'em done and my back was screaming at me! I really wanted this thing out of the way today but it's not gonna happen until tomorrow. I only have 2 of them left to do so I should able to wrap this this up, once and for all, tomorrow. As long as Murphy gives ANY kind of a break. If I had known the Murph-man was gonna be so stinking diligent on this one, I'd have gone with gender benders for sure.
Sheesh! What a nightmare this little one hour project has become!

[Guitarzan]

So I took some pix before finishing up so you can see what I had, what I went up against, and what I ended up with. A sort of before and after in one frame. Well, 3 frames actually. 2 views of the work in progress and shot of the difference in length between the old connector and a new one. Of course if I'd known it was gonna this much of pain in the schmendle, I would have bought gender benders and been done with it. But...it is what it is now. It came out OK and someday when I finally decide to sell off all of my stuff, I'll just hope the first guy that looks at it isn't some kind of Bogen purist who's repulsed by the modification I made. Anyway, when you consider how much those Impedance transformers can fetch on the bay these days, I could sell those off one by one and end up with more money than I originally paid for the entire amp 5 or 6 years ago. I think I got it for 50 bucks, if I remember correctly. Might've been 60, but I'm thinking it was 50.


Thanks again guy, for all the input, and for just being interested. Now I can clean up my mess, and get back to the Maggie. That, after all was the original project. I was only trying to get the Bogen outta the way without just pushing it aside with the top cover off etc.


Sheesh!

[jjasilli]

Nice job!


My suggestion is, don't worry about the purists.  It can't be had both ways.  I think you either treat an item as a collection piece and keep it pristine; or put it to use.


My wife watches Antique Road Show and always likes to tell me when some vintage guitar is worth a lot more money, because it's all original.  I'm lucky to have some nice guitars but they weren't vintage when I got them, and I bought them to play them.  That means re-frets, new nuts and saddles, etc.  over the years.


With working amps you need to replace caps, etc. anyway.  It can't stray pristine and be used.  I think the greater good is keeping these gems in useful service.  The work you've done is 1st-class and respectful to the integrity of the amp and its design, while better adapting it for current use.   

[Willabe]

Nice job!

Yeah it is!   

My suggestion is, don't worry about the purists.  It can't be had both ways.  I think you either treat an item as a collection piece and keep it pristine; or put it to use.

I agree, it's a Bogen and your gonna use it. 

Terry Kath of Chicago used Bogen amps for his guitar rig, even after they made it big and he could have played through any amp he wanted..